Summary:
zstd files are downloaded and used as part of JNI build, but are left behind even after doing a `make clean`. This PR updates the `clean` target to remove these zstd files as well.
Closes https://github.com/facebook/rocksdb/pull/2365

Differential Revision: D5123537

Pulled By: sagar0

fbshipit-source-id: a8f355da5ba961aa89d5852e35751ffc35de03ea